The present invention relates to controlling a load voltage for a field device in a way that has stable behavior under shut down circumstances.
A load voltage controller for a field device may be used in a manufacturing process to monitor the operation of the process and to actuate process variables of the process. Typically, actuators are placed in the manufacturing field to drive different process control elements, such as valves and sensors. Further, transmitters are placed in the manufacturing field to monitor process variables, such as fluid pressure, fluid temperature or fluid flow.
Such transmitters are coupled to a control loop and transmit process information over the control loop to a centralized controller which monitors the overall operation of the manufacturing process. The control loop may be implemented as a two-wire loop carrying a current that provides power for operation of the field devices.
In such control systems, communication is typically through a field bus standard, which is a digital communication standard with which transmitters may be coupled to only a single control loop to transmit sensed process variables to the central controller. Related communication standards are described in ISA 50.02-1992 section 11. Another standard is HART(copyright) which allows digital communication to be superimposed on a 4-20 mA process variable signal.
An important aspect with respect to control systems of the type outlined above is intrinsic safety. When field devices are located in a hazardous area without explosion-proof equipment, the electronics in the field device itself should be intrinsically safe. Intrinsic safety means that the electronics must be designed in a way that no sparks and no potential heat from the components may occur even if one or more electronic failures occur at the same time. Intrinsic safety is achieved through additional protection elements designed to protect the electronics under a fault condition. Depending on the specific type of applicationxe2x80x94e.g., the explosive type of gases used within a manufacturing processxe2x80x94there exist different requirements for a certain design, different specifications for the protection elements, as well as different certifications.
FIG. 1 shows further details of the field device being connected to a field bus. In particular, FIG. 1 shows elements of a single field device 10 adapted to a field bus 12.
As shown in FIG. 1, the field bus 12 may be represented by an equivalent circuit diagram with an ideal voltage source 14 and a resistor 16 to model AC voltage impedance and to fulfill intrinsic safety requirements for spark protection, current limitation and power limitation in the hazardous area.
As also shown in FIG. 1, the field device 10 is connected to the field bus 12 via a first wire 18 and a second wire 20.
As also shown in FIG. 1, the field device 10 divides into a Graetz-diode-bridge 22, a discharge protection diode 24, a capacitor 26, a DC/DC converter 28 and a load 30. The load 30 is shown as a resistor and describes an actuator operating on, e.g., a valve used in the manufacturing process or any other control element, or a transmitter being adapted to measure manufacturing process variables as outlined above.
As also shown in FIG. 1, a connection between the field device 10 and the field bus 12 is achieved by connecting the nodes 32 and 34 of the bridge arm to the first wire 18 and the second wire 20, respectively. Further, the node 36 connecting the anodes of the two upper diodes in the Graetz-diode-bridge 22 is connected to ground 38 while the node 40 connecting the cathodes of the two lower diodes of the Graetz-diode-bridge 22 is connected to the anode of the discharge protection diode 24. The cathode of the discharge protection diode 24 is connected to the capacitor 26 which itself is connected across the input terminals of the DC/DC converter 28.
Operatively, the Graetz-diode-bridge 22 in the field device 10 serves to avoid any polarity sensitivity with respect to the signal of the field bus 12. Further, the Graetz-diode-bridge 22 is provided to support intrinsic safety together with the discharge protection diode 24, e.g., when the discharge protection diode 24 fails. In other words, it is not necessary to consider polarity of the voltage on the field bus 12 when connecting the field device 10 to the field bus 12.
Also, the discharge protection diode 24 is inserted to increase the intrinsic safety of the field device 10 through blocking the discharge of capacitors comprised in the field device in the event that a circuit element of the field device fails. In other words, discharge of the effective capacitance in the field device 10 into the control loop is inhibited by an isolation network consisting of the Graetz-diode-bridge 22 and the discharge protection diode 24 with three levels of redundancy.
Further, operatively the capacitor 26 connected to the cathode of the discharge protection diode 24 serves to stabilize the input voltage Ui to the DC/DC converter 28. Therefore, if the input voltage Ui to the DC/DC converter 28 breaks down due to lack of energy supply from the field bus 12, the capacitor 26 is discharged. For this reason, the supply of energy to the DC/DC converter 28 and therefore also the load 22 will be maintained over a certain time through discharge of the capacitor 26 until the voltage across the capacitor 26 is too low to drive the DC/DC converter 28 and therefore to drive the load 30.
FIG. 2 shows further details of the field device 10 shown in FIG. 1. In FIG. 2, the elements to the left side of the capacitor 26 are summarized into an equivalent current source 42.
As shown in FIG. 2, the DC/DC converter 28 may be realized as a charge pump adapted to map the input voltage Ui, or equivalently the input current, to the DC/DC converter 28 into a suitable output voltage Uo, or equivalently the output current, over n stages.
Further, the circuit diagram shown in FIG. 2 adds an operational amplifier 44 and a Zener diode 46 to the circuit elements shown in FIG. 1.
The cathode of the Zener diode 46 is connected to the output of the DC/DC converter 28 while the anode of the Zener diode 46 is connected to ground.
Also, the positive supply of the operational amplifier 44 is the output voltage Uo of the DC/DC converter and the negative supply of the operational amplifier 44 is ground potential. The output terminal of the operational amplifier 44 is connected to the output node of the DC/DC converter 28. Further, one input terminal of the operational amplifier 44 is also connected to the output of the DC/DC converter 28 while the other input terminal of the operational amplifier 44 receives an externally supplied reference voltage signal Uref.
Operatively, the operational amplifier 44 and the Zener diode 46 are alternatives to limit the output voltage Uo of the DC/DC converter 28 to either the voltage reference Uref of the operational amplifier 44 or to the Zener diode voltage Uz of the Zener diode 46. When the Zener diode voltage Uz is higher than the voltage reference Uref of the operational amplifier 44, the Zener diode 46 may operatively achieve protection against higher voltages for overheat protection of circuit elements if the operational amplifier fails. Also, the Zener diode 46 may prevent sparks when capacitors with higher voltages are short-circuited.
As outlined above, operatively the charge pump maps the input voltage Ui or equivalently the input current into a suitable output voltage Uo or equivalently output current over n stages. As is commonly known in the art, the behavior of an ideal charge pump may be described using the following model:
From the above identified model, it may be derived that, since the output Uo of the DC/DC converter 28 is controlled and the input voltage Ui is predefined through the energy supply via the field bus 12, the control variable at the input terminal of the DC/DC converter 28 must be a current. This is the reason why the power source coupled to the capacitor 26 is modeled as an ideal current source 42. Further, it may be assumed that during shut-down the ideal current source 42 is switched off, as will be discussed in more detail below.
Further, operatively, the operational amplifier 44 or the Zener diode 46 acts as a bypass drawing current when the output voltage Uo of the DC/DC converter 28 exceeds the voltage reference signal Uref. As long as Uref greater than Uo, the current supplied by the DC/DC converter 28 flows only over the load 30. Otherwise, when Uref less than Uo, the surplus current supplied by the DC/DC converter 28 flows over the bypass, i.e. from the output of the operational amplifier 44 to ground and/or over the Zener diode 46.
However, as shown in FIG. 2, the capacitor 26 storing energy to bridge an operational shut-down condition is charged with the input voltage Ui of the DC/DC converter 28. In case of power fault, the discharge current of the capacitor 26 and the voltage across the capacitor 26 will therefore be supplied to the input of the DC/DC converter 28 and then be subsequently processed over n stages in the charge pump of the DC/DC converter 28. This discharge of the capacitor 26 decreases directly the output voltage Uo of the DC/DC converter 28 until a limit value thereof is reached that causes a reset of the circuit. In other words, the output voltage of the DC/DC converter Uo decreases with the voltage across the capacitor 26 so that the reset point is reached very fast. As a result, the period during which the discharge current of the capacitor 26 may be used as an energy source for the load 30 during a shut-down/power fault on the field bus is reduced.
In one general aspect, a load voltage controller for a field device includes a DC/DC converter adapted to receive a power supply voltage signal on a power supply line of the field device for conversion into a load voltage signal and subsequent supply to a load of the field device; a comparison unit adapted to compare the load voltage signal with an externally supplied load voltage reference signal; and an energy buffer adapted to store supplied energy when an output signal of the comparison unit indicates that the load voltage signal exceeds the load voltage reference signal.
The load voltage controller may be used to provide a constant load voltage for a predictable time after shut-down of the energy supply to the DC/DC converter in order to run a procedure for shut-down under stable circumstances. In some implementations, the overall capacitance in the field device may be reduced while the buffer time is increased.
In general, the buffer time available for the field device is not determined by the input current/voltage to the DC/DC converter but through the capacity of the energy buffer and the amount of energy supplied thereto. Therefore, through the provision of the energy buffer, the operation of the field device becomes more reliable and stable under a shut-down condition. Still further, it is possible to avoid any loss of energy in case the output voltage of the DC/DC converter exceeds the load voltage reference signal, which may result in an overall better energy management in the field device.
As a result, the load side provision of an energy buffer and the supply of energy thereto can be combined with a multilevel redundancy network isolation scheme such as is discussed above with respect to FIG. 1 without requiring any modifications of the hardware at the input side of the DC/DC converter.
In addition, the energy buffer may be adapted to supply energy to the load of the field device when the output signal of the comparison unit indicates that the load voltage signal is lower than the load voltage reference signal. This leads to the advantage that the energy stored in the energy buffer is only withdrawn when the load voltage signal is actually lower than the load voltage reference signal. Therefore, the energy buffer is always supplied with energy to the maximum extent without waste of energy.
In another general aspect, a switching unit is connected between the output of the DC/DC converter and the energy buffer and processes the output signal of the comparison unit to establish a connection between the DC/DC converter and the energy buffer when the load voltage reference signal exceeds the load voltage signal for supply of energy from the energy buffer to the load of the field device; disconnects the DC/DC converter and the energy buffer when the load voltage reference signal equals the load voltage signal; and establishes a current path between the energy buffer and the power supply line of the field device when the load voltage reference signal is lower than the load voltage signal for supply of energy from the energy supply line to the energy buffer.
The provision of a current switching unit between the output of the DC/DC converter and the energy buffer allows achievement of a three state operation of the load voltage controller through continuous comparison of the load voltage signal and the load voltage reference signal in the comparison unit. Energy may be stored in or supplied from the energy buffer without delay in compliance with the comparison result. Also, in case the load voltage signal exceeds the load voltage reference signal, the DC/DC converter supplies energy to the load while simultaneously storing energy in the energy buffer through the power voltage signal, thus achieving a parallel operation in the load voltage controller for preparation for subsequent shut-down conditions.
A bypass may coupled across the energy buffer for overvoltage protection to ensure operability of the load voltage controller and to draw the overcurrent not being supplied to the load.
The comparison unit and the switching unit may be realized in an integrated manner as a first operational amplifier having the power supply voltage signal on the power supply line as positive supply and the load voltage signal as negative supply and receiving the load voltage reference signal and load voltage signal as input signals, respectively. This solution allows the use of minimal amount of extra hardware, i.e. using only one additional operational amplifier and one capacitor and Zener diode. Therefore, the overall capacitance in the field device is reduced while the buffer time is still long. Also, a relatively constant output voltage is assured for a predictable time after shut-down of the input energy supply to run a procedure for shut-down handling under stable circumstances. Here, the predictable time depends on the output load current in relation to the additional capacitor of the load voltage controller. Another particular feature of this implementation is that the load voltage controller always uses the difference between the input voltage to the DC/DC converter and the output voltage for energy supply to the energy buffer.
In some implementations, the DC/DC converter may be a charge pump with at least two stages, the charge pump being adapted to provide at least one intermediate voltage signal with respect to each stage having a voltage level lying between the power supply voltage signal and the load voltage signal, respectively; and the comparison unit and the switching unit are realized in an integrated manner as a second operational amplifier having the power supply voltage signal on the power supply line as a positive supply and the at least one intermediate voltage signal of the charge pump as a negative supply, and receiving the load voltage reference signal and the at least one intermediate voltage signal as input signals, respectively.
This implementation allows further fine-tuning of the predictable time after shut-down, since this time also depends on the voltage level of the charge pump stages that may be discharged. In other words, access to the different stages of the charge pump permits modification of the predictable time for stable operation after a shut-down.
The comparison unit and the switching unit may be realized in an integrated manner as a third operational amplifier; the DC/DC converter may be a charge pump with at least two stages, the charge pump being adapted to provide intermediate voltage signals with respect to each stage having a voltage level lying between the power supply voltage signal and the load voltage signal, respectively; a first multiplexer is provided for selection of either the power supply voltage signal on the power supply line or one of the intermediate voltage signals as positive supply of the third operational amplifier; a second multiplexer is provided for selection of either one of the intermediate voltage signals or the load voltage signal as negative supply of the third operational amplifier; and the third operational amplifier receives the load voltage reference signal and the output signal of the second multiplexer as input signals, respectively. The reference voltage of the third operational amplifier is switched in relation to the positive supply voltage of the third operational amplifier.
This implementation achieves the same advantages as outlined above with respect to the implementations using the first operational amplifier and the second operational amplifier, respectively. Also, use of a first multiplexer and a second multiplexer permits modification of the positive supply and the negative supply of the third operational amplifier and also the signal to be compared with the load voltage reference signal so that an overall more flexible adaptation of the predictable time after shut-down, e.g., in dependence of external requirements is achievable.
Similar advantages may be achieved employing other systems in conjunction with techniques and methods such as those implemented by the systems described above.
In yet another general aspect a computer program product directly loadable into the internal memory of a load voltage microcontroller includes software code portions for controlling load control techniques when the product is run on the load voltage microcontroller. The microcontroller measures the voltage of the buffer capacitor and determines the multiplexer channel.
These techniques are also provided to achieve an implementation of the method steps on computer or processor systems. In conclusion, such implementation leads to the provision of computer program products for use with a load voltage control microcomputer.
The programs defining the functions of the present system can be delivered to a computer/processor in many forms, including, but not limited to information permanently stored on non-writable storage media, e.g., read only memory devices such as ROM or CD ROM discs readable by processors or computer I/O attachments; information stored on writable storage media, i.e. floppy discs and hard drives; or information convey to a computer/processor through communication media such as network and/or telephone networks and/or internet via modems or other interface devices. It should be understood that such media, when carrying processor readable instructions implementing the described concepts represent alternate implementations of the system.
The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.